We begin this hour in Kiev, the capital city of Ukraine. Riot police have surrounded the main square, the site of a week's long anti-government protest. Members of one of the main opposition party say armed troops raided their headquarters today and seized computer equipment. Police are also threatening to enforce a court order to drive protesters out of city buildings they've been occupying.

NPR's Corey Flintoff is Kiev and joins us now. And, Corey, you've just come back from a walk around the square. Describe to us what's happening there.

COREY FLINTOFF, BYLINE: Well, one of the most dramatic things about this confrontation, Audie, is that it's taking place in a wintry city where we've just had four or five inches of fresh snow. So you have to picture all of this against a white backdrop. Thousands of people have been occupying the Maidan, the main square here, for weeks. They're protesting the government's last minute refusal to sign a key trade agreement with the European Union.

I've been here for a week now, and there's been virtually no police presence at all. But today, all of that changed. There's busloads of police in full riot gear, started blocking all the entrance to the square. And by nightfall, they fortified those positions with trucks and buses, so they can now control any vehicles going in and out. And that's crucial because the protesters have been bringing in food and water and firewood to support their encampment. Police have also dismantled some of the protest camps and barricades that were built away from the main square and near some government buildings.

CORNISH: Corey, has there been any violence, any clashes between police and protesters?

FLINTOFF: Well, not so far, but we could be very close to violence here. I'd like to play some tape that I recorded a little while ago in the street next to Kiev's national museum.

(SOUNDBITE OF PROTESTS)

UNIDENTIFIED GROUP: (Foreign language spoken)

FLINTOFF: Just to set the scene here, about 200 police officers had formed a line of five or six men deep just all the way across the street. They planted their metal shields on the pavement to form a barrier, and they were literally nose to nose with the group of protesters, about equal numbers on each side. The protesters pressed against the shields of the troops and yelling at them, just exhorting these young troopers. So it was a very tense situation. I asked our Ukrainian fixer, Tanya Batanova, what they were saying.

TANYA BATANOVA: A lot of things, kind of abusive without bad words, but telling these guys that you shouldn't stand here against your people.

FLINTOFF: Then a guy standing nearby heard us speaking English, and he had this warning for us.

UNIDENTIFIED MAN: Just got some information that the American embassy urges everyone who's here to leave this place simply because they're expecting some violence to go off.

FLINTOFF: And so it was at this point, when someone just had to throw something and it would've turned into a very bloody melee. But as it was, a couple of opposition people's deputy showed up, and they very courageously walked down the line, between these two sides, parting them like the Red Sea. And at that point, the police turned and moved back a few hundred yards and the situation was diffused.

CORNISH: And as we said, Corey, one of the main opposition parties claims that police raided their offices. Just a short time left. What have you learned about that?

FLINTOFF: Well, a spokesman from the Fatherland Party said armed men broke into the party headquarters, they seized computer equipment and then left. So this raid on the headquarters might be an effort to get evidence that could be used to charge some of the party leaders who've been very active in the protest.